1/8 Buggy-Reasons 80% of all conversions doesn´t work. [Archive]

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NIC

09-13-2006, 10:16 AM

Hi,

1. Not enough voltage.
2. Cheaper and lower efficient motors.
3. Gearing.

1. Power is Voltage*Amps. If the voltage is too low you will need to draw more amps to reach your desired power output.
Let´s say you will need a 1000 watts to be satisfied with the power for your track, could aswell be 800 or 1200W. Going with a higher voltage set-up will draw less amps to reach that power.
An extremely simple and not very accurate early calculation would be to compare a 4S set-up to a 5S set-up.

1000W/14,8V=67,6A for 4S.
1000W/18,5V=54A for 5S.

Going from a 4S-pack to a 5S set-up with the same cells would require the pack to deliver 20% less amps to reach the same power (4S to 5S=20%).
Everyone knows that figure will be higher because the 5S pack will maintain an higher voltage because of the lower amps draw from that pack. And the loss in connectors and esc will be less with lower amp draw also.
Going with a higher voltage set-up will give you a better "starting point" and is essential for a higher powered buggy.
A 4S set-up in a 1/8 buggy will not work as good as a 5S. I will stick out my head and say 4S is not enough to make a good performing 1/8 buggy.

2. Motors. Here´s were the interesting stuff is.
What matters here is how cool the motor will run at my desired power output and how much the motor will work the esc.
Let´s compare a higher efficient motor-90%, to a 80% efficient motor.
Say we wanted 1000W output for our track, could likewise be 800W or 1200W, it doesn´t matter really for this example.
A motor is only as powerful as it can disipate or transfer away heat.
I will borrow some numbers of the Plettenberg BigMAXXimum from Plettenbergs homepage.

V A RPM W in W out % Torque
18,1 70,3 37688,4 1271,2 1141,6 89,8 28,9

On 18V and pulling 70A that motor will produce and give you 1147W with an efficiency of a very good 90% between 30-70A.
That motor will produce 130W of wasted energy wich translate into heat, 1271W in-1141W out=130W. Let´s say that 130W of energy would heat up that motor to a temp. of 70C or 160F when we completed our run. It doesn´t matter how long we ran, it´s just for comparison.
That 70C or 160F we are ok with and consider a fair and safe temp to run our motor at.
Now what happens when we just alow the 80% efficient motor to disipate the same 130 watts and therfore heat up to the same temp.?
Will we get the same power output of 1147W or maybe just 1000W or even less.........?
1270W into the 80% motor will give you 1016W out from the motor but with 260W of wasted enery !
We only wanted 130W of wasted enery and heat to not go over 70C or 160F motortemp.
Could we at least get 800W out from the motor ? No !
1000W into the motor would give 800W out but still 200W of losses. To much.
600W then ? No ! 750W in and 600W out, still 150W of wasted energy.
650W in with an efficiency of 80% is 520W out with our targeted 130W limit of wasted energy. That is a big differens.
The Plettenberg wich we used earlier will only produce 50W of wasted heat at a 520W power output. Were as the 80% efficient motor will not make our goal of 70C if it pulls more than around 35A continious (650W/18,5V). In a car it´s mostly alot of peaks currents followed by really low currents.

Alot of the conversions done have the batteries cold and the motor screaming hot and the esc either very hot or constantly going into thermal shutdown. That has mostly to do with gearing wich is the last point.

3. Gearing. A 1/8 buggy is really tough for a motor because of the large rotatingmass off the centerdifferential, front and rear driveshafts and the input gear to the front and rear diffs. This large rotatingmass comes directly after an relatively low gearreduction, 3,5:1 with 46T spur and 13T pinion.
Gearing a buggy with larger pinions will skyrocket the amps the motor will pull because of all the starts and stops the motor has to perform with all that mass during a race. This leads to high temps on the motor and esc especially, but even on the batteries if they are just average ones.
Try always to gear the buggy with the smallest pinion to make it as easy as possible for the motor and esc to drive the buggy.

A good point to shoot for is to have a motor spinning at 40000 rpms with the batteries used and gear it with a 10T pinion. This will make the load on the motor as low as possible, 4,6:1 ratio for the centerdiff compared to 3,5:1 with a 13T pinion, while still matching the nitros in speed.

Never use a motor with a higher kv then 2200 and compensate that with less voltage, that set-up will either be too weak and slow or heat up the motor too much if you try to gear up to have some decent speeds. If using an "XL" motor with a kv of 2200 and higher and gear it for higher speeds on lower cell count will only draw too many amps wich will heat up the esc and the batteries too much. The power will be "wrong".
My advise is not to use any 2-pole "L"-motors for a buggy-Hacker L, Feigao L, Lehner 1930:s and so on. They don´t have enough torque in my opinion. If you aren´t running on a VERY small track that is, with speeds under 50km/h or 30mph.
Instead use 5S with a 2200kv motor to have 40000 rpms and start with a 10T pinion wich will be enough most of the time speedwise, and use a 1700-1800kv motor if you run 6S with the same pinion.

NIC

pullinteef

09-13-2006, 05:37 PM

Thanks for the excellent write-up. I'm sure -many- people will keep this as a good reference. Do you or does anyone know if there is an appreciable difference in the efficiency of Plettenberg vs. Neu motors? I'm trying to decide between the two for my next upgrade from the Feigao 9L. Thanks again.

DrMotor

09-13-2006, 06:56 PM

That was a long post!
I agree with most of it, but some points may be open for discussion.

1. Voltage
Switching loss appears to be a killer for some ESCs at higher voltage.
Which ESCs are good at 5s and above?

Series or parallel cells; consider the following:
You have two 3s packs with connectors on each and you are going to discharge the packs over a given time.
If you hook the connectors up in series or parallel, the connectors will see the same current and have same loss.

2. Efficiency
A low cost motor with a low cost cooling system (heat sink and fan) can be more powerful than a high quality motor with poor cooling.

3. Gearing
The kinetic energy stored in the diffs, shafts and wheels is independent of pinion size as long as vehicle speed and wheel size is constant.

Too high or too low gear may be equally bad. Maybe the reasons why you recommend using as small pinion as possible can be found under point 1: High switching loss at high voltage. If using a smaller pinion allows you to run more of the time at 100% throttle, then the switching loss is much reduced. What do you think?

Shawn Palmer

09-14-2006, 12:48 AM

In 3 posts, you guys have managed to hit on some topics that would fill an entire textbook in order to be fully explained. This is one of the reasons I love the electric hobby (flight, boats and cars) so much. It's a challenge. There's homework involved. There's always an uncharted application somewhere to experiment in and share successes and failures. Unfortunately, these same reasons scare off more folks than they bring in, until things are "stable" and "proven".

Bravo NIC - as a whole, you've hit the jackpot with your initial post.
I've been in electric flight and/or cars for almost 20 years, and my mantra has always been: "Voltage is your FRIEND." I remember the absolute explosion of activity and innovation in the flight market when lipos first came on the main scene about 5 yrs ago. The standard of 7 cells went to 3s lipo almost overnight, and that alone fueled the entire industry to enormous levels of growth since then. I'm fortunate enough to have seen and participated in that era, and now I get to actually SHAPE the same transition through my work at Castle Creations for the car market. But back on track - you're absolutely correct that 5s is better than 4s, and 6s is even better, and 10-12s in cars is not as far away or as farfetched as it seems! There aren't many hard and fast generalities when it comes to electrics, but "more voltage is better to do the same amount of work" (watts) always holds true. The only limiting factors are the ESCs to handle the volts, and the motors with low enough Kvs to make the best use of them properly.

Your take on motors is generally in the right direction. More often than not, an extremely high quality motor will outperform a lesser quality motor of the same physical size and Kv. The problem comes in when the question is raised of: "what's a high quality motor, and how does it compare to the rest?"
Pullinteef raises an extremely valid question asking about the efficiency difference between Plett and Neu. The efficiency #'s given by motor manufacturers basically mean diddly squat unless you know the amp and volt level that # comes from. It's simply a peak efficiency at X amps and Y volts. If one motors 95% efficiency comes at 5v and 30A, and anothers 85% efficiency comes at 16v and 60A - which would you want in your buggy? Power loads in cars are rediculously difficult to predict or plan for. Accelleration in an 18th scale car can draw momentary loads of well over 100A! Scale that up to a 4wd 8th scale buggy on good traction, and you're in the range of thousands of amps. What's happening now generally, is the motor in the buggy is either A) overcapable and running cool and happy (and therefore the ESC and batts are happy too) or B) undercapable and trying to operate WELL out of it's efficiency range and therefore drawing tons of amps, so it gets hot itself, along with the ESC and batts. The problem in cars is that there's not much grey area to play in, and one man's "perfect combo" can be another man's "money pit of equipment blowing up" just because of a different tire diameter, slightly bigger pinion, or even just a different track layout.

And here's where voltage comes in yet again - with higher voltage to do the same "work" - the amps go down, and because amps = heat, the temps come down, and even a marginal or really bad (motor or) system on 4s may be kinda OK on 5s, and might be really darn good on 6s. It just depends on how broad and high the efficiency curve (yes - it's a curve - not just a flat figure) is for that particular motor.

Pullinteef:
I've had experience with both Plett and Neu. I'm always up front with my biases, and I know Steve Neu personally. I also know that everyone I've talked to running one of his motors has been blown away by their performance. Personally speaking, I dont think there is another brand available that comes close to Neu's motors when you compare them apples-to-apples. I also know from Castle's testing that my personal conclusions have been borne out (to a surprising degree) by actual dyno testing. There are others on this board running Neu motors with the same results, and one of them now holds the land speed record for R/C cars. :-)

On the gearing point, I have to say that if you've got a completely capable motor in place, gearing should only serve to fine tune top speed for the particular track/application. If going up 2-3 teeth in pinion is the difference between a happy and a too-hot system - you might just be lacking in overall motor capability a bit. I see this a lot with helicopters actually. The system that "demands" gearing within one or two teeth to be happy is left in the dust by the system that's OK with a wider range of gearing options, and of course that's ultimately acheived by a bigger/more capable motor.

The 40K rpms at full throttle figure may be more important than you think....
8th scale buggy "big boys" are looking into electrics, and they want them to perform as similarly in rpm as possible to their glow engines so as not to have to re-gear the whole standardized buggy for it. 35-40K is what they're looking for in a motor, and then determining cell count, ESC capability, and motor Kv in order to get there. The question right now that the ESC makers, motor makers, and battery companies are all asking is: Will things standardize at 4s, 5s, or 6s to get that 35-40K? That's where things are litterally in your hands right now - lower voltage = lower consumer pack costs, and lower ESC costs. If a proven system can be found at 4s, that's where it'll land. Personally, I'm a self-professed voltage junkie and think it'll settle out for the time being at 5 or hopefully 6s, and then move on to 10s in a few years.

DrMotor:
I disagree with you slightly on the switching losses thing. Good hardware design, software execution, and quality parts useage bring that down to almost a non-issue (for Castle anyway). We've paved the way in ESC design for many years and are regularly copied. The companies having switching loss issues should look into some better FETs, FET drivers, or improved software. We have ESCs running up to 12s (in aircraft) right now, and up to 20s in the near future. We design around partial throttle use (where switching losses occur) and actually were rated #1 for partial throttle efficiency in a magazine a little while back.

Bad motor w/good cooling vs good motor w/no cooling:
It's a little bit nit-picking, but there are a lot of instances where I dissagree with that strongly. I'll give you that a motor with a good heat sink and forced air fan will sink a LOT of heat, and run much much cooler than without them. But the first question that comes to mind is WHY? Why add complexity, weight, and power draw just for the ability to hopefully not burn out an outclassed motor for the application? If a motor is running that hot so to need all the extras, it's in a very poor place for average efficiency and runtime as well as overall power, and will suffer no matter how much cooler you're helping it run. A better motor with no cooling (and no added complexity) will have more punch, runtime, less load on the ESC and batteries, and won't be in danger of melting down every run or heaven forbid - if the fan stops working. Another scenario I've seen myself: I've de-magged outclassed motors with no cooling help, and the can temps were only 120F or less (it generally takes over 200F to demagnitize Neo rotors depending on the gause rating). When you ask more from a motor than it can really deliver, the excess eddy currents within the internal electromagnetic circle of flow will heat the rotor directly (and Nickel plating enhances these eddy currents and the heating effect) and can demagnitize it from either internal direct heat, or just plain excessive electromagnetic force and render the motor useless. Again - nit-picking to a degree, but there is nothing to be gained in my mind from using a not-quite-capable motor, and adding complexity to the system just to keep it from going critical.

Kinetic energy:
True - at a constant speed, everything equalizes. Unfortunately, in a car on a track - constant speed is the exception rather than the rule. There is continual accelleration, braking, feathering, jump approach and landing, etc., etc. With a "bad" motor, all these things really add up to a lot of excess heat for everything in the system when the motor is out of it's efficiency range.

More time at 100% throttle:
This has everything to do with motor Kv, voltage, and gearing matching for the track, and very little to do with switching losses specifically. A system put together for 60mph constantly being driven at 30mph or less will get just as hot or hotter as if you were actually driving at 60mph. Speed = watts, and watts generally means more amps. Now you're absolutely right on the point of if you were geared/motored/voltaged for a 30mph max in the same car, it's gonna take a lot less watts (and therefore amps) to get there, and you'll run a whole lot cooler. The specific reason why I can explain in detail if you wish, but doesn't involve switching losses, but DOES involve switching itself and how any ESC really works.

To get back to the beginning guys - I don't want to come off as a know-it-all, because I DON'T. If I had all the answers, I'd be kicking Pavidis and Tebo's butts every week with an electric 8th scale. (Or paying them BIG $$ to win with their own under the Castle sponsorship flag) I enjoy the conversation because it's cutting edge electric stuff, and we're ALL still learning what it takes to make one of these beasts competitive with a poorly understood and myth-ridden embryonic power system style. I hope this thread continues for a good long time, because I'm looking foreward to hearing more and learning more about what we need to do to make this easy and affordable in order for electric 8th to go full on mainstream.

Shawn

glassdoctor

09-14-2006, 03:52 AM

If I had all the answers, I'd be kicking Pavidis and Tebo's butts every week with an electric 8th scale. (Or paying them BIG $$ to win with their own under the Castle sponsorship flag) .

Shawn

We aren't far from that right now Shawn and I think you know it. ;)

I'm seeing this first hand.... my Mamba/Neu powered BL 1/8 buggy is lethal in the hands of a good driver on a local level. I have no doubt this buggy can compete at any level. And this is without much time spent in setup R&D. With more time and testing it will only get better.

4s lipo IMO is a good standard for now but I would like to see 8s as a standard. Smaller lighter motors and ice cold motor/esc/batt. (relatively... batts like some heat :) )

MikeBusch2

09-14-2006, 03:57 AM

Can't.........read........anymore........
http://stef.rominet.free.fr/gifs/stock_gifs/exploding-head.gif

(it'd be funnier if the image would post, lol.)

NIC

09-14-2006, 12:55 PM

Hi,

Shawn I´m glad and kinda hoped you would come on here and write exactly what you did :). I thought about writing a more indept and detailed post but desided to keep it simple and short enough to describe the "problem".
And to be fair I´m really slow at typing and due to my lack of vocabulary I coulden´t explain it in good english. I agree with you fully in what you wrote, it was like hearing myself.

I see it everywere and hear people talk about slapping some batteries to a 1/8 nitrobuggy and never have to deal with the hassle that comes with nitro. What they usually end up doing is giving themselfs new different problems that doesn´t lead to a more enjoyable and effective use of their buggy with less wrenching and more fun actually racing their buggys wich was the main reason they did the conversion in the first place.

Reason I didn´t mention 6S as much and gave the example with the Plettenberg and 5S was that to my knowledge their isn´t a controller that have what you need from a car controller and at the same time can handle 6S.
Maybe Shawn and Castle can do something about that......?

I can sense it in the air that everyone out there, the 1/8 buggy conversion, trucky and montertruck guys really are looking for a higher voltage esc with all the functions of a good car controller. And I think it will come in the shape of the Monster MAXX. Question is how much voltage it will handle ?

Another reason that I didn´t mention 6S much is the batteries, the weight and size of them. I think it would be ok to use 6S-3200mAh of the higher quality cells like the Kokams and others, but I think that the safetymargin would be a bit to small for feeling comfortable. 6S of the 4800-mAh Kokams for example I would feel ok with, but then the weight would be on the edge of running out of mechanicle set-up with springs as the main cause but also tires.
And the physical size of the pack might be hard to fit in a good way in a buggy.
The weight and size of a 3S-3200 pack but with 4800mAh would be optimal. It might take a year or so for that kind of cells to be available though.

I would like to see 6S as the standard because with 5S of the highest quality cells the power is enough, then just use 6S for a little more power ( ;) ) and reducing the amps abit while keeping the weight of the car equal left and right as the majority of all conversions have a split battery. Then the simplicity of having the same number of cells in both batterypacks with regards to setting up the charger (chargers) for the same number of cells. Also with two 3S packs with 4800mAh used in series in a 1/8 buggy, it would be perfect to use one of those, a 3S-4800 mAh, in pretty much all other 1/10 vehicles such as a touringcar for example.

NIC

kufman

09-14-2006, 01:14 PM

The specific reason why I can explain in detail if you wish, but doesn't involve switching losses, but DOES involve switching itself and how any ESC really works.

Please do!!! :)

Shawn Palmer

09-19-2006, 08:30 PM

Kufman:
It's easiest in an airplane application, but works the same for cars too:
In the most basic (but still 100% accurate) sense, any speed control is no more than an on and off switch. When it's "on" it's providing full current and voltage to the motor, and when it's "off" it's providing no current and voltage to the motor. Just like a light bulb on a wall switch - you get 60 watts of light or none.

In an airplane, you have a certain size propeller on the motor, and it will draw a certain amount of amps at full throttle. If it draws 50A at full throttle, that's what each "on" pulse is providing - 50A.

So what happens at partial throttle? Well, if you flick a wall switch on and off several thousand times per second and the amount of time it spends on and off are equal, the observed result is only 30 watts of light. But the bulb is drawing 60 watts when it's on, and 0 watts when it's off. Same with an ESC. at 1/2 throttle with the above airplane example, each on pulse is still 50A, and each off pulse is still zero. If the time of those pulses are equal, the observed current is 25A, and the prop rpm is about 1/2 of full throttle.

So essentially, any ESC is always operating at full throttle current and voltage. There may be less pulses of on than off and the observed result is less speed/rpm than full, but the individual units of operation are still at full throttle current and voltage.

So when we apply the above to cars, we start to see why it's not a good thing to turn down the throttle EPA to make a system slower. If you've got a 70mph system (speed = watts, and speed also = heat) that you need to only be 40mph for the track - it's still operating on the current it needs to get to 70mph, even though the observed top speed is slower. That's why we stress GEARING to bring the system down to the speed you need, and not just using less throttle.

glassDr.:
"We aren't far from that right now Shawn and I think you know it."
Well don't go telling everyone!!!!!!

NIC:
"Maybe Shawn and Castle can do something about that......?"
Maybe.......just maybe..... :-)

"Also with two 3S packs with 4800mAh used in series in a 1/8 buggy, it would be perfect to use one of those, a 3S-4800 mAh, in pretty much all other 1/10 vehicles such as a touringcar for example."
Jackpot!

Shawn

H-Trainer

10-23-2006, 05:31 AM

This is an excellent thread, I've never found so much well-funded and interesting information in only 5 posts before. thanks a lot

Looks like I'm waiting for capable 6S car controllers to come out and the batteries to evolve a bit further.

You mentioned having the motor run at 40'000 RPM. I'm reading that many motors can turn up to 60'000 or even 80'000 RPM. Is there a gain in efficiency when running the motor slower than its limit or do you just wanna have some room for going up with the voltage?

What would you prefer in a 1/8 buggy and which has more torque out of the following:
- Lehner 2230 (44 mm diameter, 61 mm length, 1700 W, 80 A)
- Lehner 1950 (36 mm diameter, 76 mm length, 1800 W, 100 A)
Say, a thick vs. a long motor with roughly the same power.

Looking forward to you answers. thanks in advance

Regards,

Thomas

pauster

10-24-2006, 10:22 AM

Hi Thomas

I am not an expert ... but looking at the two option you list

- Lehner 2230 (44 mm diameter, 61 mm length, 1700 W, 80 A)
- Lehner 1950 (36 mm diameter, 76 mm length, 1800 W, 100 A)

the first would mean 21.25 V (6s ?) and the second 18 V (5s ?)
- and looking at this thread the higher voltage is preferable (power
is voltage times current, or W = V x A)

IF the batteries, ESC, motors, cabling etc. play along

Just an observation, I might be totally wrong ...

Patrick

H-Trainer

10-24-2006, 11:29 AM

Patrick, thanks for your reply. It seems that you calculated 1700 W / 80 A = 21.25 V etc.

These are the ratings for maximum power and ampère. I'm not sure if these were practically measured at the same time, making your calculated voltage a fact.

And BTW, 2230 and 1950 are size indications, not kV. Both have a rev limit of 65'000 RPM and about 5-40 windings.

My question was meant to ask for general pros and cons of A) fat, short motors and B) slim, long motors, so I just chose two from Lehner with roughly the same power.

pauster

10-24-2006, 12:21 PM

Hi Thomas

not sure but if I were you I'd go by the motor curves for the anticipated RPM etc ranges - Lehner publishes them under the heading "Allgemeine Technik" at

http://www.bk-electronics.com/bk.html

The first two digits seem to relate to motor diameter, and the second two length indicators. You probably knew that already :o)

Patrick

pauster

10-24-2006, 12:43 PM

An interesting thing when you look at the motor curves - the voltage stays almost constant and the amperage increases under load. The rpm only drops slightly.

With real batteries the voltage might drop also a bit more under load, making the rpms drop a tad more.

I would choose a kv model and a set of gears so that the relevant speed range I want for the vehicle falls into the highest efficiency range of the motor (actually you'd need to go across several diagrams for different throttle settings).

You'd need a good estimate of the power requirements of the vehicle class, based on experience, say a 1000W motor for an E-Revo with a top speed of 45 mph ... or 500W for a 1/10 scale 2WD Losi etc ???

Is this somehow realistic ? I am curious what the best way for sizing and selecting motors and ESCs and batteries is. I am a RC newbie after all - I have been out of it for 20 years and want to start again ...

Sedri

10-24-2006, 12:51 PM

Wow, this looks like the thread I was looking for - all of you have shared a lot of excellent theory and possible combinations and considered the effect of the different variables involved in setting up a 1/8 buggy. Here's my pitch, and a chance to start testing these ideas:

Let's hold some of the variables constant, and test each variable in a "real-world" way, if you like. For instance, I have a 1/8 "buggy" chassis setup, but it's actually an onroad conversion; the Kyosho mp 7.5 chassis which is used in the Inferno GT (posted elsewhere under "Lipo 4S GT")

I have a 1500W 540XL Feigao, a 4000maH, 14.8 Lipo pack and a BK 9920 with a max draw of ~100A.

Right now I still have the stock 13/48, 17/43 2-speed transmission, and I want to convert to a single speed which puts the motor, batteries and SC in their "optimum" operating range, with the motor @40,000 - 42,000 rpm's. With the 2-speed, it had so much torque to the wheels (as it has right now with the 1st gear) to make even 2/3 -3/4 throttle on take off uncontrollable, and the 60+ top speed in 2nd gear simply pushes the chassis way past its limits, and is not particularly practical in a typical on-road racing situation.

Any "single-gearing" suggestions to take best advantage of the beforestated brushless setup? I'm ready to put in an order for the pinion and spur gear, but I don't have a clear choice yet.

I'm thinking about a "46" spur gear (hardened steel is available) and a 16 or 17 pinion.

NIC

10-24-2006, 01:50 PM

Hi H-Trainer,

The 2230 will for sure have more torque then a 1950 with similar kv, say a 1950/8 vs. a 2230/12. The 2230 will pull more amps.

NIC

glassdoctor

10-24-2006, 06:14 PM

You could run a 3s lipo to tame the car.

or can you somehow disable 2nd gear? (take out the shoes in the "clutch" of the 2 speed or something to that effect)

If you replace with a single speed, I would recomend the kyosho plastic 46T gear.

Sedri

10-26-2006, 12:40 PM

Mmm, but I don't want to just disable 2nd gear - there is still the friction/efficiency issue of "four gears" meshing versus only one pinion and spur gear in a single speed setup, and I want as much efficiency as possible. The power lost in excess friction and rotating mass becomes significant at the high rpm's of the brushless motor.

Wow, and run 3S? Hehe, I prefer the entertainment of figuring out how to gear the motor and set up the chassis to handle the power of 4S instead of just detuning the power of the motor itself - it was the lure of "too much power" that made me convert from nitro to brushless in the first place!

If I want a car that has completely controllable power matched to the chassis, I'll just take my nitro GT for a spin, hehe!

jhbronx

10-29-2006, 07:44 PM

A system put together for 60mph constantly being driven at 30mph or less will get just as hot or hotter as if you were actually driving at 60mph. Speed = watts, and watts generally means more amps. Now you're absolutely right on the point of if you were geared/motored/voltaged for a 30mph max in the same car, it's gonna take a lot less watts (and therefore amps) to get there, and you'll run a whole lot cooler. The specific reason why I can explain in detail if you wish, but doesn't involve switching losses, but DOES involve switching itself and how any ESC really works.

Shawn
How would it run hotter if it's running at half throttle (30 mph) but it's geared for 60 mph and it's pulling the same amps?

jhbronx

10-29-2006, 07:49 PM

Looks like I'm waiting for capable 6S car controllers to come out and the batteries to evolve a bit further.
Thomas

Why isn't the Mgm Compro 160 amp, 24 cell brushless esc capable? Is the amperage that it would pull in a eigth scale buggy too great even with a high voltage 5s, 6s pack and a low Kv motor?

H-Trainer

10-30-2006, 08:45 AM

What are the specifications on the Mgm Compro 160?

I was referring to Nic's post:
... Reason I didn´t mention 6S as much and gave the example with the Plettenberg and 5S was that to my knowledge their isn´t a controller that have what you need from a car controller and at the same time can handle 6S.
...
I can sense it in the air that everyone out there, the 1/8 buggy conversion, trucky and montertruck guys really are looking for a higher voltage esc with all the functions of a good car controller. And I think it will come in the shape of the Monster MAXX. ...

jhbronx,

it possibly is like this because most BL motors (if not all) have the highest efficiency at a certain RPM range. So it would run less efficient at 40% of its maximum revs and thus generate more heat.
This all is assuming that Shawn knows what he does five days a week. ;)

I believe that a couple years ago, most BL motors and/or ESCs for airplanes would blow up because of eccessive heat if you ran them at mid-throttle for too long.

jhbronx

10-30-2006, 05:14 PM

What are the specifications on the Mgm Compro 160?
jhbronx,
it possibly is like this because most BL motors (if not all) have the highest efficiency at a certain RPM range. So it would run less efficient at 40% of its maximum revs and thus generate more heat.
This all is assuming that Shawn knows what he does five days a week. ;)
I believe that a couple years ago, most BL motors and/or ESCs for airplanes would blow up because of eccessive heat if you ran them at mid-throttle for too long.

The MGM Compro is 160 amps and can handle up to 8s lipo.
In regards to brushless motors heating up at half throttle, what exactly is causing the heat build up within the motor?

Sedri

10-30-2006, 07:59 PM

Dear Forum,

This may seem like a simplistic answer the the question above (jhbronx's), but we could think of it this way -

Any electric motor which is paired to its ideal voltage/ampere source will rum most efficiently (theoretically; not realistically) when it is simply directly wired to the battery; that is, take the motor, connect the battery directly to the motor leads and there it is, "zero loss", or maximum efficiency.

We know that's not real-world, though...we need to be able to control the speed of the motor (throttle).

Those of you who were in R/C in the 80's certainly remember the 3-step speed controllers. How did they produce throttle step? By "dissipating" power thru resistors. Modern electronic speed controls, though in a more efficient, "cleaner" way, still need to "step down" maximum potential power to the motor for modulation of throttle/motor speed, but I think it is still clear to say that the battery/speed control/motor system only sees maximum efficiency - that is, battery potential converted to motor performance - when the connection is direct, or when the speed control (mechanical, electronic, whatever) is not modulating the voltage or current in any respect, but instead making a "100%" connection between the power source and the motor.

This is why speed controls and motors can still heat up at partial throttles - they are simply not utilizing the most efficient transfer of the battery's power to the motor.

This is why, as NIC argued above, one should shoot for the balance of battery, motor and gearing that allows the complete system to operate at maximum efficiency as much as possible for the application needed...if a motor, for example, has an ideal max rpm of 40,000 @ 16.60-16.80v, it's good to gear for the motor to be able to spend most of its time at or in the ballpark of that speed. Undergearing, and you are percentaging the throttle; overgearing, and your 'lil motor is grunting and burning itself up trying to hit its potential.

jhbronx

10-30-2006, 09:39 PM

O.K., but what's causing the resistence, at partial throttle, in a brushless motor? The way I understood Shawn_Palmer describe it, It's like an on and of switch effect, with partial throttle being a longer off phase then the on phase of the switching, if I'm understanding it correctly then I'm still confused as to where the extra heat is being generated at partial throttle. :confused:

jhbronx

10-30-2006, 09:45 PM

Kufman:
So what happens at partial throttle? Well, if you flick a wall switch on and off several thousand times per second and the amount of time it spends on and off are equal, the observed result is only 30 watts of light. But the bulb is drawing 60 watts when it's on, and 0 watts when it's off. Same with an ESC. at 1/2 throttle with the above airplane example, each on pulse is still 50A, and each off pulse is still zero. If the time of those pulses are equal, the observed current is 25A, and the prop rpm is about 1/2 of full throttle.

So essentially, any ESC is always operating at full throttle current and voltage. There may be less pulses of on than off and the observed result is less speed/rpm than full, but the individual units of operation are still at full throttle current and voltage.

Shawn
This is how I thought modern speed controls worked.

Sedri

10-31-2006, 12:33 PM

Jhbronx,

Yes, modern ESC's are much more efficient than "resistor-type" SC's, that's no argument - they work in a completely different way - yet, in spite of that, they are still "working" - that is, at partial throttle settings, they are still manipulating the power source's transmission to the motor. Not by dumping power into resistors to lower voltage or amps, but rather by switching on and off.

Nevertheless, this electronic switching on and off, even though it is electronic and not mechanical (and thus "looks" to us as though nothing is really moving or being wasted, still is "moving" - electrons though, with the "gate" action of the MOSFET's and like it or not, even at the atomic level, this switching still requires energy to accomplish, and in an imperfect (but getting closer) world, there is still the byproduct of heat generated from the FET's switching on and off. Electricity moving simply has the byproduct of heat; it's a physical phenomenon. The FET's require some energy to "work"...you could think of it as friction heat, but electron-generated friction. Remember, in the wires, FET's, etc, there ARE actually things moving around - the electrons.

My brushless setup uses a high frequency ESC, what some people call "squealers" or "whistlers"

(http://www.modelboatmayhem.co.uk/Common/Electrics/Speed%20controlers.htm)

which is the sound of the MOSFETs switching at partial throttle settings - and mine has never gotten "hot" - just comfortably warm.

I don't know if this clarifies anything about your original question; I think the main point here is that, even though modern ESC's are becoming more efficient, they still have SOME (although smaller than previous ESC types) loss and inefficiency - that's just unavoidable - and this loss translates physically as heat.

electraflyit

10-31-2006, 06:05 PM

The way I understand the ESC and heat is this. As said earlier the ESC turns on and off many times a second, at a high throttle setting its 'On' a lot longer than its 'Off', and at a lower setting more 'Off' than 'On'. The Motor sees this as and increase and decrease of Voltage (changes the speed). The Heat in a ESC is generated by AMPS. Amps are generated by Load. Maxium load is a full throttle start with no wheel spin. At this position current is only limited by wire resistance, ESC resistance and batterys, as the motor speeds up the Amps fall until the motor is at its max RPM with the current load(it reaches a balance). At partial throttle there are less amps being drawn because less volts mean less amps. Even with a little less effiency in the ESC at partial throttle less Amps mean less heat.
Eddie

mtucker

11-01-2006, 01:08 AM

All good information. To throw in my two cents..... the majority of heat created in a ESC (or any pulse width modulator) is

1. I squared R losses AKA copper losses AKA the on resistance of the FETs with lots of current flowing as well as the connections and wiring.
2. Switching losses. The time it takes the transistors to switch on/off and off/on. When they are partially on they can lose a lot energy even though it is for a very short time. Like Mr. Palmer said, the swtiching losses can be minimized (by using MOSFETs that have low gate capacitance and by driving the gates with high current devices so the transition time is as short as possible).

That was excellent information NIC. I think proper gearing is more than half of the battle in getting long and reliable runs out of 1/8 conversions. Right now the 1/8 applications are pushing the limits on most speed controls.... especially if you are trying a conversion on a budget. Of course what fun would it be if everyone had a brushless 1/8 buggy, truck, or truggy? :D Half of the fun is running something that not everyone else has.

Matt

jhbronx

11-01-2006, 06:14 PM

I still don't understand how less current can cause more heat than full current.

H-Trainer

11-02-2006, 06:52 AM

As mtucker (and other people) explained, there are switching losses from "hacking" the current (on-off-on-off...) to reduce the output power. Think of electricity as a flow of water. If you close and open the pipe several thousand times per second, the water molecules are accelerated and decellerated all the time and hitting the valve when it's closed. This is some sort of friction and causes heat.
Contrary to this, a valve that is open all the time lets through more water, but has much less resistence (friction).

The real ESC and motor experts can feel welcome to supplement (or correct) my explanation.

Thomas

mtucker

11-02-2006, 02:45 PM

That sounded like a good explanation to me. At 80% throttle you have losses from swtiching and the MOSFET's conducting. At 100% thottle you no longer have swtiching losses.
That falls apart however with brushless motors since your ESC can't just apply constant DC to your brushless motor. The ESC has to keep the switching going even at full throttle (the nature of brushless motors).

The biggest problem with overgearing and using partial throttle is the motor doesn't perform nearly as efficiently if it is over geared. I read a great article in a RC plane magazine explaining how over gearing.... or propping in this case... caused the motor to run inefficiently. Unfortunately I don't remember enough of it to summarize.
The effect is quite noticeable on airplanes where you can go from a 8" prop to a 9" prop and suddenly your motor goes from warm to untouchable.... even though you are flying about the same speed.

Matt

jhbronx

11-02-2006, 05:19 PM

Reinhard

11-06-2006, 08:26 AM

for the partial load-thing:

http://www.kraeuterbutter.at/Bilder2/CRONO/Crono_Messwerte/Crono_Bilder.jpeg

i have a 1:8 scale buggy
Lehner 1930-8 (star-configuration)
1895 mikro
and use it with 2s lipo (20-25km/h for childrin)
3s
4s (best compromise)
and 5s

all with same gearing (11,5)

here i have some onboard-telemetrie-datas:

--- with 4s -------------------------------------------------------
Ampere, Volts and the Motor-Temperature:
http://www.kraeuterbutter.at/Bilder2/CRONO/Crono_Messwerte/Crono_Ampere_Temp_Volt.gif

Power and Motor rpm:
http://www.kraeuterbutter.at/Bilder2/CRONO/Crono_Messwerte/Crono_Watt_Drehzahl.gif

driven on gras, ambient-temp was 22°C
motor-temp started at 27°C because it was not the first batt on that day
speed around 46-48km/h
within 11minutes 2200mAh from the 4400mAh-batts were used

--- with 5s -------------------------------------------------------
Volt, Ampere and Motor Temperature
http://www.kraeuterbutter.at/Bilder2/CRONO/Crono_Messwerte/Bild1_5s_Prolite_V_A_temp.jpg

Watts and Motor-Rpm
http://www.kraeuterbutter.at/Bilder2/CRONO/Crono_Messwerte/Bild1_5s_Prolite_W_rpm.jpg

motor-rpm at around 30.000rpm
950Watt Peak (far enough)
amps: peaks only in 50A reagion
within 12min 2250mAh used (from 4200mah availabe)

================================================== =======

Conclusion:
look at motor-temps !!!
at the 5s-Setup (without regearing) the temps went up to 98°C after 12min
(batts only halfe-empty !!)
so: if i would have continued driving, the motor would have been damaged for SURE !

at 4s after same time the temp is "only" around 65°C

nevertheless:
on 4s i used an average of 192Watt
on 5s i used an average of 178Watt

NEVERTHELESS the 5s-Setup run a lot hotter...
BECAUSE: i used more partial load to handle the car in my yard

smhertzog

11-16-2006, 08:22 AM

This is one of the most informative posts I have ever read on brushless motors gearing and esc's and how they effect each other. :teacher: :D

Other reasons 1/8th scale conversions fail which I dont think anyone has metioned is the fact that many neglect to use a slipper clutch (center diff buggies and truggies mostly). And or they use the stock metal spur. They think if it can work on my 3+ horse power nitro it will be fine on my 1+ horse power brushless. Nitros have a clutch which takes out alot of the shock and do not have near the linear torque of a xl type motor. Neglecting to use a slipper clutch is at best very hard on the drive train and motor. If you dont break stuff alot more often you will definately wear it much quicker. While using a metal spur on the other hand may waste the spur and pinion in only a matter of a few runs.

ElectricThunder

11-16-2006, 05:22 PM

Reinhard

11-17-2006, 06:29 AM

of course you are right..
but i had no other Pinion
(i have no pinions, i use the Slipper-Clutch from the nitro-car where the clutch is removed on a mill so that only the pinion is left..
the hole in the pinion has 8mm size
--> thats standard for propellers...
so i bought for 2 Euro a alu-Prop-"thing" (my english, my english.. right word missing)
where i mount the pinion like a prop !
works without problems..

i thought, that it would run without regearing, because i was not high with currents at all..
BUT: i use the motor in star-configuration (which means, that the motor can not handle that much current as in triangle-configuration)

will equipp the car soon with 5s1p FePO4 (costs only 50 Euro)
2300mAh is enough, and 5s FePo4 compare to 4s Lipo when it comes to voltage

kufman

11-17-2006, 01:00 PM

so i bought for 2 Euro a alu-Prop-"thing" (my english, my english.. right word missing)

Prop adapter, like these

http://www2.towerhobbies.com/cgi-bin/WTI0095P?FVSEARCH=prop+adapter&FVPROFIL=++

pauster

11-17-2006, 04:05 PM

Reinhard

11-18-2006, 10:11 AM

@pauster:
http://www.eagletreesystems.com/MicroPower/micro.htm

pauster

11-18-2006, 10:58 AM

@pauster:
http://www.eagletreesystems.com/MicroPower/micro.htm

cool stuff, even with GPS :winner: but a setup seems to cost $200 - $400 or more ... but the engineer in me is really tempted :D

Reinhard

11-18-2006, 11:01 AM

the micro-Logger i use costs only 80Euro

GordonFreeman

11-20-2006, 01:25 AM

I'm sure I've seen it somewhere, just can't find it. Not a new idea, but using a low Kv outrunner in an 1/8 scale replacing the center diff with dual output to front and rear diffs. Is this a horrible idea (no center diff)? Perhaps a motor like this:
http://www.horizonhobby.com/Products/Default.aspx?ProdID=EFLM4060A
It would need a longer shaft for dual output.

Advantages:
1. Lighter (no diff, pinions, etc.)
2. Less mechanics (no gear mesh worries)
3. No "diff-out" under braking.
4. Better natural cooling? (natural airflow of outrunner)

Disadvantages:
1. No center diff
2. Less sealed motor
3. Can't change gearing (other than cell count)

Thoughts?

Reinhard

11-20-2006, 01:57 AM

Mr. Constructor did it with an King of dirt 4WD 1:8 buggy:

look here:
http://rcsaga2.free.fr/Guest/Constructor/Big/King%20Of%20Dirt%2001%20640.jpg

at this page (were you can find a lot other projects from Mr. constructor):
http://www.rcsaga.com/constructor/1.htm

tcolesen

11-20-2006, 10:16 AM

That would be an interesting idea, but for offroad I'd be worried. If you land off of a jump with the throttle on, you can pretty much say bye-bye to your diff gears.

ducati777

11-20-2006, 11:14 AM

I gotta put in my 2 cents... All this partial throttle stuff doesn't make any sense at all. Brushless ESC's have to switch at all throttle levels. The one thing I don't know is lets say you're using a standard 3 wire brushless motor, and the ESC has to power the first coil. At partial throttle is it a series of short bursts to the one coil, or does the one coil just get 1 short burst, compared to full throttle, which would be a longer burst?

If its just one, then the number of switching isn't dependant on throttle position like we've seen on brushed ESC's. Which would throw out the partial throttle causing heat arguements.

Reinhard

11-20-2006, 11:34 AM

maybe somebody could also explane the thing which is so called:
"Active free wheeling circuit"

important thing for partial load...
(Kontronik Jazz has, but does it also use at RC-car-Mode ???
--> active free wheeling circuit makes the first 10% of the rpm not controllable)

glassdoctor

11-20-2006, 11:37 AM

The esc has to switch on/off much more at partial throttle... more work=more heat.

I'm not claiming to know exactly how and why....

Castle says full throttle creates less heat than partial throttle, which is why they say not to just turn down the EPA to tame the system.

Reinhard

11-20-2006, 12:17 PM

other example:
on the old 3SL40-6-18:
Kontronik said: it can handle for 10seconds 70A in a hotliner..
BUT ONLY, if you switch it on and off by an switch..
and not slowly with the stick...
because if you are at such currents in partial load you fry the controller..

H-Trainer

11-20-2006, 04:26 PM

Gordon, there's a thread on the subject of center BL designs, outrunners etc.: http://forums.radiocontrolzone.com/showthread.php?t=182802

About the partial throttle: I'd assume that commutating the current to the different coils at full throttle is much less work than switching on and off plus the commutating at partial throttle.
As I explained with the valve in a water pipe. If you just reroute the water to different pipes, the molecules keep flowing, just changing direction. But closing and opening the valve several hundred/thousand times per second causes turbulence, resistance, heat... .
I'd appreciate if a BL expert could confirm this, but I believe it makes sense anyway.